1. Field of the Invention
The invention relates to the field of printing systems, and in particular, to color conversion of image data that includes various levels of gray, such as text.
2. Statement of the Problem
In color printing, a printer prints input image data onto a tangible medium, such as paper, by converting colors in the image data to a color space of the printer. For example, the printer may have a CMYK color space that is used to represent the image data using various levels of Cyan (C), Magenta (M), Yellow (Y), and blacK (K). Before the image data is printed, the image data is converted to the typically smaller gamut of the CMYK printer. Because of this smaller gamut of that color space, precise representation of the image data is generally not possible as information is lost or misrepresented in the conversion process.
Adding to the complexity, the input image data may be a perceptual color space, such as CIEL*a*b*, that is visualized as three dimensional color space, where every color that humans can see is uniquely located. Though the CIEL*a*b* color space is a perceptual color space, it is not a perceptually uniform color space as the Euclidean distance in the space does not correspond to the perceptual distance. For example, the magnitude of the perceptual color difference generally depends upon the color location and the changing direction in chroma and hue. CIEL*a*b* increasingly overstates the magnitudes of perceived chroma differences. The human visual system (HVS) is sensitive to the change of chroma in the neutral color area and insensitive to the change of the chroma in a highly saturated color area. The CIEL*a*b* color space is also non-uniform regarding hue angle in that the thresholds of visual tolerances are a function of hue angle. If the non-uniformity of the CIEL*a*b* color space in chroma and hue is examined from another perspective, CIEL*a*b* colors have different characteristics at different locations.
A color conversion model can be generated using this data to convert color values of the CIEL*a*b* space to the CMYK space. Generally, the relationship between CMYK color space and the CIEL*a*b* color space is nonlinear due to the interactions of cyan, magenta, yellow, and black planes, leading to more complex color conversion processing. Moreover, the printer may misregister the colors and improperly print them. Generally, misregistration regards the printer's inability to precisely align different color planes. For example, to produce a certain gray value, the printer may drop a certain amount of cyan, magenta, yellow, and black on top of each other. When the drops are not correctly aligned, the misalignment causes the drops to blur. Misregistration is particularly problematic during the color conversion of gray values in that it leads to less than desirable gray representations in certain cases (e.g., unclear or “fuzzy” text, barcodes, etc.). That is, the printed gray representations of items such as text, barcodes, lines, and the like often appear unclear after standard color conversion and printing when compared to the original image data. Accordingly, there exists a need to convert colors that remain esthetically pleasing while ensuring that the converted gray values representing lines, text, barcodes, etc. are sharp and vivid.